1. Field of the Invention
The present invention generally relates to a DC/AC inverter controlling system suitable for a solar cell type power supply unit. More specifically, the present invention is directed to a controlling system capable of deriving maximum AC power from a DC/AC inverter coupled to a solar cell type power source by controlling an operation point of the DC/AC inverter over a wide output voltage range, namely by a maximum power point tracking function.
2. Description of the Prior Art
Very recently, to save artificial energy obtained from the earth and also to avoid environment pollution, it becomes more and more popular to utilize so-called "natural energy sources" such as solar energy and wind energy. Various power controllers for solar cell type power supply units have been developed and are commercially available, which are described in, for instance, "Comparisons of Maximum Power Tracking Strategy of Solar Cell Output and Control Characteristics using Step up/down Chopper Circuit" written by T. Ohnishi et al., T. IEE Japan Vol. 112-D, No. 3, 1992, and Japanese Patent Publication No. 61-2202 published on Jan. 23, 1986.
FIG. 1 represents a schematic block diagram of one conventional controlling apparatus for controlling a DC/AC inverter coupled to a solar cell type power source.
In general, DC voltages derived from a solar cell are changed in response to amounts of solar energy given to this solar cell when deriving maximum power from the solar cell. As a consequence, a DC/AC inverter for such a solar cell type power source is controlled by changing the DC input voltage of this inverter in order that DC input power to this inverter becomes a maximum value, namely maximum DC power can be obtained from the solar cell type power source.
In the conventional control apparatus used for a solar cell type DC/AC inverter, a DC voltage derived from a solar cell 1 is applied to charge a capacitor 3 via a reverse current stopping diode 2, and also to a DC/AC inverter 4. An AC output from this DC/AC inverter 4 is connected to an AC power line via a filter constructed of a reactor 5 and a capacitor 6, whereby DC power generated by the solar cell 1 is inverted into AC power which will then be supplied to this AC power line.
In this case, the DC output power of the solar cell 1 is calculated by a multiplier 10 based upon both of a voltage detected by a voltage detector 8 and a current sensed by a current detector 9. A power control to increase and decrease a DC voltage command value is performed by a power control unit 11 in response to this DC output power from the solar cell 1 in accordance with an algorithm as shown in FIG. 2 in such a manner that this DC output power becomes maximum. This power control operation is repeated for a predetermined time period.
Upon commencement of such a power control, a judgement is performed whether the power is increased or decreased by comparing the latest power with the previous power, and then a decision is made that the present voltage command value is increased or decreased, depending upon an increase or a decrease in the preceding voltage command value. For instance, if the present power is increased at a step ST-21 (YES) and the preceding voltage command value is increased at a step ST-22 (YES), then the present voltage command value is increased at a step ST-23. To the contrary, if the present power is increased at the step ST-21 (YES) and the preceding voltage command value is not increased, namely equal to, or decreased, as compared with the present voltage command value at the step ST-22 (NO), then the present voltage command value is decreased at a step ST-25. Furthermore, when the present power is not increased at the step ST-21, the similar judgement is made of the preceding voltage command value at a further step ST-26 in this flow chart shown in FIG. 2. As a result, such a voltage command by which the DC output power from the solar cell 1 may become maximum, is issued.
Referring back to the circuit diagram shown in FIG. 1, a voltage control unit 12 compares this voltage command with the output voltage from the solar cell 1 to obtain a voltage deviation value, and outputs a current command to a current control unit 13 so as to reduce this voltage deviation value to zero. Thereafter, this current control unit 13 compares this current command with an AC current detected by a current detector 7 to obtain a current deviation value, and then performs the PWM (pulse width modulation) control of the DC/AC inverter 4 in order to reduce this current deviation value to zero.
It should be noted that although the current control unit 13 has such a function by which an AC current command is produced in synchronism with a voltage phase of the AC power line and the AC power line is driven at a power factor of 100%, since this function has no direct relationship with the present invention, no further explanation thereof is made in the following descriptions.
As previously explained, the above-described conventional inverter controlling apparatus intends to control the DC output power from the solar cell 1 at maximum values. However, there is a problem that the inverting efficiency of the DC/AC inverter 4 is varied in response to the DC input voltage thereof, and therefore the AC power derived from the DC/AC inverter 4 is not always supplied to the AC power line at its maximum efficiency, which AC power is supplied to the AC power line, or loads. In general, it is known that the higher, a DC input voltage of a PWM-controlled DC/AC inverter becomes, the lower, an inverting efficiency thereof becomes. As a consequence, a DC voltage "V.sub.1 " at which DC power "PDC" becomes maximum is not coincident with another DC voltage "V.sub.2 " at which AC power "P.sub.AC " becomes maximum, as clearly represented in FIG. 3. In FIG. 3, the first DC voltage "V.sub.1 " is higher than the second DC voltage "V.sub.2 ".
Moreover, there is another problem in the conventional power control algorithm as explained in FIG. 2. When the amounts of solar energy incident upon the solar cell 1 are changed, there are some dangerous conditions that the voltage command values are issued irrelevant to the maximum output power points. More specifically, as represented in FIG. 4, after the maximum power control is executed under such conditions that the output characteristic of the solar cell 1 is "C.sub.1 " and the output voltage thereof is "V.sub.1 ", and the voltage command value is decreased, if the amount of solar energy incident upon the solar cell 1 is increased and then the output characteristics of this solar cell 1 are changed from C.sub.1 to C.sub.2 and C.sub.3, the voltage command control is established along a direction of A-D-E, irrelevant to the maximum output points B and C. Subsequently, this voltage command control might be effected from the E point to the C point. It this case, some time delays may be produced until the maximum power control can be achieved.
The present invention has been made in an attempt to solve the above-described various problems, and therefore has an object to provide an DC/AC inverter controlling system used for a solar cell type power source, capable of increasing an efficiency of the overall controlling system, while supplying maximum AC power to an AC power line, or load, and furthermore capable of improving a follow-up (tracking) characteristic of the system when amounts of solar energy given to the solar cell type power source are changed. As a consequence, the maximum AC power can be continuously supplied from the DC/AC inverter to the AC power line.